1. Field of the Invention
This invention relates generally to offshore floating structures used in the exploration and production of offshore minerals, such as semi-submersible vessels, tension leg platforms, and spar type platforms, and more particularly to offshore floating structures having vertically spaced buoyancy capsules enclosed within an open cross-braced jacket framework or truss support structure.
2. Brief Description of the Prior Art
In the following discussion, the words “jacket” and “truss”, as used herein, refers to a welded or bolted open frame structure formed of slender tubular members. A conventional truss bridges between three or more buoyant vertical column structures that stabilize a semi-submersible vessel at the water surface when floating with respect to wind, wave, current and other horizontal loads. Horizontally disposed truss structures are known as “truss pontoons”, and vertically disposed truss structures are known as “truss columns”.
Floating vessels and semi-submersible floating vessels, such as floating production platforms, storage and offloading vessels, tension leg platforms (TLPs) and SPAR structures, are commonly used for oil well drilling, oil production and living and working quarters. It is desirable to design floating structures with minimum heave (vertical) oscillations to waves in the ocean environment.
Conventional semi-submersible vessel or platforms comprise three or more large diameter vertical columns that are spatially separated and connected at their bottom ends by large horizontal pontoons. The deck structure is above the water with a sufficient distance between the still water level to the bottom of the deck to allow waves to pass across the columns without impacting on the bottom of the deck. The center of gravity of the entire semi-submersible vessel is generally high due to large deck loads above the water. The columns and pontoons of modern semi-submersibles are usually constructed of shells formed of thin metal plates backed with welded stiffeners, frames, stringers, bulkheads and stiffeners. The pontoon and columns have compartments for ballast, voids and storages. The columns and the pontoons provide the necessary buoyancy to the semi-submersible to accommodate the structure, equipment and live loads.
Since the center of the gravity of the semi-submersible vessel is much higher above the center of buoyancy of the lower hull, the acting moments due the wind, wave and currents horizontal forces are restored by the semi column water plane areas and moment of inertia of the water plane areas. The size and the spacing of the columns provide these two stability parameters. Thus the semi-submersible design is called a “column-stabilized floating unit”. The semi-submersible vessel is a free-floating vessel compliant to wind, waves and currents. It is moored and/or dynamically positioned by powered-thrusters.
Semi-submersible vessels have good sea-keeping behavior with respect to waves and have large deck areas and carrying capacity. However the current and wind resistance results in higher positioning. A semi-submersible vessel is sensitive to the variable deck loads and its stability is limited. Semi-submersibles have large heave motions (vertical oscillation of the floating vessel) when subjected to waves and consequently a dry-tree oil production system is not feasible. In the case of a drilling semi-submersible, downtime of drilling may occur when the heave motion is not feasible for drilling.
The tension leg platform (TLP) was introduced as a new concept based on a semi-submersible design with a deck, columns or caissons and pontoons. The vertical heave of the tension leg platform (TLP) is reduced by tendons attached between the lower hull and the seabed. The tendons are always maintained in tension with the excess buoyancy designed into the lower hull. The application of a TLP is limited to water depths of say 5000 ft., over which, the tendons' vertical neutral period enters the energetic portion of the wave heave forces of the TLP. Secondly, the size of the lower hull increases the economics to always maintain tension in the tendons with respect to the increased water depth.
The SPAR is currently used in relatively large offshore oil production applications. However, the SPAR has limitations in deck area and payload due to its size. The SPAR is a single large vertical column or caisson buoyant structure that floats vertically and is stationed on location by moorings and has excellent heave performance like a TLP. Unlike the semi-submersibles, the stability of the SPAR is not dependent on the water plane area and the moment of inertia of the water plane area. The stability of the SPAR is provided by lowering its center of gravity below its center of buoyancy. Thus, the vertical buoyancy acts upwards above the center of gravity and total weight acts at the center of gravity below the center of buoyancy. Ultra deepwater posses problems to the riser tensioning system and limits the applicability of the SPAR. Mooring of the SPAR in deepwater over 5000 ft. also posses a serious problem. The effectiveness of the mooring and handling are reduced in such water depth. The SPAR also poses vortex shedding vibration problems when the vertical single column hull becomes extremely slender.
Recent SPAR designs employ a number of outer buoyancy cylinders surrounding an inner shell all interconnected at an interval along the length of the SPAR caisson. The SPAR structure is stable because the center of gravity is maintained below the center of buoyancy of the whole structure in its operation.
Typically, the SPAR hull vertical caisson structure is built in a Far East or European shipyard due to inadequate fabrication facilities in United States shipyards. The SPAR structure is then dry-towed to a location, for example, the Gulf of Mexico, commissioned quayside, and then wet-towed to the operating site and upended. The moorings are hooked up after upending on the site and topside modules are lifted onto the deck by heavy lift-crane installation-vessels.
Two types of oil production trees are used with floating production vessels or structures; dry-tree production and wet-tree production, depending upon the vertical motion of the floating vessel to the swelling of the seas due to wind. Dry-tree production is used, for example, in the deepwater offshore oil production platforms deployed in the Gulf of Mexico. Tension leg platform (TLP) and SPAR type structures are typical offshore oil production platforms that are used for dry-tree production in deepwater applications. Semi-submersible platforms are used for wet-tree productions. The dry tree platforms require a larger deck area than the wet tree production platforms.
In the case of a dry-tree production semi-submersible platform, the hull, along with a semi-finished upper deck, is typically built in a Far East shipyard and dry-towed to a location, for example, the Gulf of Mexico. The remaining topside modules are built in a U.S. shipyard and mounted on top of the deck near the shore. The platform is then towed to the operating site and hooked up to the moorings.
In either the case, the submerged hull of the SPAR or semi-submersible floating platforms are predominantly built in shipyards outside of the United States and dry-towed to U.S. waters. The cost and time involved in the fabrication and transportation of the hulls are significant in the project of an offshore oil production development, and in some marginal production fields, the conventional fabrication and transportation process is not time and cost effective.
Another important consideration is the engineering design and fabrication time involved in producing the shell-type hull floating-vessels. Generally, the engineering time is little over a year and the fabrication time is over two to three years. The conventional hull is designed to have thin shells formed of metal sheet or plates backed with welded stiffeners and transverse frames. These types of thin walled stiffened shells are susceptible to fracture and crack propagation that might lead to large catastrophic failure. Periodic inspections and maintenance are also required. Thus, the maintenance cost is increased for the shell-type hull floating vessels.
Horton, U.S. Pat. No. 5,558,467 discloses a spar-type deep water offshore floating apparatus for use in oil drilling and production in which an upper buoyant hull of prismatic shape is provided with a passage longitudinally extending through the hull in which risers run down to the sea floor, the bottom of the hull being located at a selected depth dependent upon the wind, wave, and current environment at the well site, which significantly reduces the wave forces acting on the bottom of the hull, a frame structure connected to the hull bottom and extending downwardly and comprising a plurality of vertically arranged bays defined by vertically spaced horizontal water entrapment plates and providing open windows around the periphery of the frame structure, the windows providing transparency to ocean currents and to wave motion in a horizontal direction to reduce drag, the vertical space between the plates corresponding to the width of the bay window, the frame structure being below significant wave action whereby wave action thereat does not contribute to heave motion of the apparatus but inhibits heave motion, the frame structure serving to modify the natural period and stability of the apparatus to minimize heave, pitch, and roll motions of the apparatus. A keel assembly at the bottom of the frame structure has ballast chambers for enabling the apparatus to float horizontally and for stabilization of the apparatus against tilting in vertical position, and taut anchor lines connected to the apparatus at a location of relatively little cyclic movement of the apparatus, the said lines being connected to suitable anchors.
Horton, III, U.S. Pat. No. 5,722,797 discloses a spar-type buoyant floating caisson for offshore drilling and production that includes means for increasing the natural period of the caisson and reducing heave, pitch, and roll without increasing the overall length of the caisson. The floating caisson has a center well through which drilling and/or production risers pass and one or more circular plates extend radially from the caisson below the water surface. The circular plates provide additional mass and resistance to environmentally induced motions and thus increases the natural period of the caisson beyond the periods of maximum wave energy, which allows the caisson to be designed with a shallower draft than a caisson without the plates that would normally be used in deep water.
Blevins et al, U.S. Pat. No. 6,206,614 discloses a spar-type floating offshore drilling/producing structure that is formed from a plurality of closely spaced vertically oriented buoyant columns on which one or more modules or decks may be placed to support process equipment, a drilling rig, utilities, and accommodations for personnel. The columns are held in the closely spaced relationship by a plurality of horizontal plates spaced along the length of the columns and vertical plates located near the bottom of the columns and near the top of the columns. The columns have a smaller water plane area than the horizontal plates. The structure includes fixed ballast, an oil storage area, and voids and variable ballast for offsetting the lighter weight of the stored oil.
Borseth et al, U.S. Pat. No. 6,375,391 discloses a guide device for production risers for petroleum production with a “dry tree semisubmersible” at large sea depths. The system includes a guide frame for one or more riser pipes, on a semisubmersible production vessel, with one or more main buoyancy member are arranged separately on at least one riser to carry the main part of the riser's weight. Each riser separately carries a Christmas tree on its top, near a main deck of the vessel. The guide frame comprises vertical main elements extending vertically downwards from the deck, through the splash zone and through the upper, more wave- and current-influenced zone of the sea. The guide frame also includes horizontal guide plates comprising vertically open cells formed of a horizontally arranged framework of beams. Lateral stabilization devices guide the risers' and the main buoyancy members' vertical movement relative to the vessel and restrict horizontal movement of the risers with respect to the guide frame. The guide plates are arranged in at least two levels on the guide frame. A lower guide plate is arranged at the lower ends of the vertical main elements', and a guide plate is arranged just below or near the splash zone. At least one main buoyancy member is held on the riser in level with, and guided by, lateral stabilization devices arranged in one or more guide plates below the upper, more wave- and current-influenced zone near the sea surface. The risers are without buoyancy elements through the splash zone, and thus are less exposed to the water forces in the upper zone of the sea.
Finn et al, U.S. Pat. No. 6,637,979 discloses a semisubmersible floating telescoping truss platform for use in marine environments. The truss is telescopingly mounted to the platform and movable between upper and lower positions with respect to the platform. At least one riser buoyancy member is telescopingly mounted to the platform and movable between upper and lower positions with respect to the platform. For each riser buoyancy member, at least one guide is attached to the truss and adjacent the buoyancy member for guiding and laterally restraining the buoyancy member.
The present invention is distinguished over the prior art in general, and these patents in particular by jacket frame floating structures that have one or more elongate vertical support columns formed of an open cross-braced jacket framework of tubular members interconnected together and at least one generally cylindrical buoyancy capsule disposed in the open framework near an upper end and at least generally cylindrical second buoyancy capsule therein near a lower end in vertically spaced relation. The buoyancy capsules may be a single upper and lower capsule, a plurality of upper and lower capsules bundled in circumferentially spaced relation, or upper and lower capsules having a cylindrical outer side wall and a cylindrical inner side wall defining a central opening extending therethrough. Alternatively, a keel tank may replace the lower capsule. The buoyancy of the upper buoyancy capsule(s) is adjustably tuned to provide a buoyant force and a sufficient water plane area and moment of inertia required for stability of the floating structure, and the water mass and weight of the lower buoyancy capsule(s) or keel tank(s) is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure with respect to its center of buoyancy according to ballast and variable or fixed loads including deck payloads, to stabilize the structure, and to compensate for different operational, environmental, survival and installation stages of the structure. The length of the upper buoyancy capsule(s) is sufficient so as to be partially submerged and allow oscillation of the trough and crest of waves within its top and bottom ends.
The design and fabrication costs of the present open cross-braced tubular jacket frame structures are significantly lower than shell-type hull fabrications. Another advantage of the open cross-braced tubular jacket frame structures is that they are easily designed and fabricated as modular structures. The jacket frame and buoyancy capsules can be fabricated at different locations in the United States and assembled at shipyards in the United States to save fabrication and transportation time and cost. The design life of such jacket frame structures is longer than the conventional shell-type hull structures. The fabrication time and cost are much lower with the jacket-type frame structures. Unlike shell-type hull structures, the jacket-type frame structure has a longer corrosion resistance life because the large surface area exposed to seawater is reduced. The resistance to fatigue is also greater because the possibility of crack initiation in a stiffened shell and stress hotspots are eliminated.